Amstrad CPC 464 Capacitor Replacement Guide

This guide documents the complete capacitor list and replacement procedure for the Amstrad CPC 464 (main PCB, cassette sub-PCB, and the CTM-640 / CTM-644 / GT-64 / GT-65 monitors that supply its 5 V power). The values are taken directly from the official Amstrad CPC464 Service Manual (1985), Electrical Parts List section.

The system unit itself runs on regulated 5 V DC supplied by the monitor — there is no AC mains inside the CPC 464 system unit and no CRT, so the system-unit capacitor work is low-risk. Monitor work involves lethal mains voltages and lethal CRT charge and is covered in safety detail below.

Safety Warning

The CPC 464 system unit contains no mains voltage and no CRT. Capacitor work on the main PCB and the cassette sub-PCB can be done at the bench with the system unit unplugged from the monitor. Standard anti-static precautions apply.

The CPC 464 monitor (CTM-640 / CTM-644 colour, GT-64 / GT-65 green) contains a CRT (15–25 kV anode), mains-rectified bulk capacitors (typically 100 µF/400 V or 220 µF/400 V), and a switching power supply. Before any work that opens the monitor case:

Power off and unplug the mains lead.
Wait 30 minutes minimum before opening.
Discharge the CRT anode through a 1 MΩ / 10 W bleed resistor from the chassis ground to the anode cap, sliding the resistor lead under the rubber boot. Do not short the anode with a screwdriver — the inrush can pit the CRT or damage the tools.
Discharge the PSU bulk capacitor through a 1 kΩ / 5 W resistor.
Verify both discharges with a multimeter before any work.

Monitor capacitor work is covered in the monitor-specific service manuals (the CPC 464 service manual covers the GT-64 and CTM-640 in pages 18–30). The detail below covers only the system unit.

Main PCB Capacitors (CPC 464, Revisions 1–2)

The main PCB carries ten electrolytic, eight ceramic (decoupling), three polystyrene (timing), and one ceramic on the AY-3-8912 sound output. Values and positions are taken directly from the service manual electrical parts list. Designators are with the IC101–IC125 main board reference scheme. Note: the C1xx range is main-board, the C3xx range is cassette sub-PCB.

**CPC 464 main PCB — electrolytic capacitors**
Designator	Value	Voltage	Function	Service-manual part #
C101, C102	47 µF	10 V	Sound output bypass at the audio amp side	1400244
C308	22 µF	10 V	Bypass on cassette input amp	20025
C309	1 µF	50 V	AY-3-8912 channel A DC-block	20062
C311	1 µF	50 V	AY-3-8912 channel B DC-block	20062
C314	1 µF	50 V	AY-3-8912 channel C DC-block	20062
C315	100 µF	16 V	Audio output coupling	20028
C318	22 µF	10 V	Bias bypass on the LA6324 cassette amp	20025
C322	470 µF	10 V	Cassette motor relay supply bypass — most-common failure	1400248
C324	10 µF	16 V	Cassette write-current shaping	20024

Note: C301, C303, C304, C306 (cassette sub-PCB electrolytics, values 100 µF/10 V and 47 µF/10 V respectively) are listed in the cassette sub-PCB table below.

**CPC 464 main PCB — ceramic capacitors**
Designator	Value	Voltage	Function
C104–C128 (25 caps)	0.1 µF	25 V	Per-IC supply decoupling. One cap across the VCC/GND pins of every IC on the main board
C307	470 pF	50 V	Sound output ceramic coupling
C310	220 pF	50 V	AY-3-8912 noise generator timing
C313	270 pF	50 V	AY-3-8912 envelope timing
C316	33 pF	50 V	Z80 clock filter
C317, C319, C323	0.022 µF	50 V	Cassette write-current shaping
C320	0.001 µF	50 V	Cassette read filter
C321	200 pF	50 V	Cassette read filter

**CPC 464 main PCB — polystyrene capacitors**
Designator	Value	Function
C302	0.068 µF	Reset RC network — sets the power-on reset pulse width
C305	0.01 µF	Cassette read low-pass filter
C312	0.001 µF	Cassette read 2 kHz tone shaping

The 25 ceramic 0.1 µF decoupling caps (C104–C128) cover one cap per IC. The exact placement varies by board revision because the chip locations move — on Revision 3 (cost-down) boards with the 40226 combined ASIC, there are fewer ICs and therefore fewer decoupling caps.

Cassette Sub-PCB Capacitors (CPC 464)

The cassette sub-PCB carries the audio amplifier IC301 (LA4140), the cassette read amplifier IC302 (LA6324), the cassette motor relay RY301, and the volume control VR301. Capacitors per the service manual:

**CPC 464 cassette sub-PCB — electrolytic capacitors**
Designator	Value	Voltage	Function
C301	100 µF	10 V	Power supply bypass at LA4140 audio amp (IC301)
C303	47 µF	10 V	LA4140 input coupling
C304	100 µF	10 V	LA4140 output coupling to speaker
C306	47 µF	10 V	LA6324 cassette read amp bypass

These four sub-PCB electrolytics are aged 35+ years and are the most common capacitor faults in the CPC 464 audio chain. Symptoms include reduced speaker volume, distorted audio, intermittent cassette read, and noise on the AY-3-8912 output.

Recommended Replacement Parts

For all electrolytics, use modern low-ESR aluminium electrolytic replacements with equal capacitance and equal-or-higher voltage rating. The original parts are 85 °C rated; 105 °C replacements add margin without cost penalty.

**Replacement summary**
Value	Voltage	Replacement type	Quantity (main + cassette PCB)
1 µF	50 V	Aluminium electrolytic, radial 5×11 mm	3
10 µF	16 V	Aluminium electrolytic, radial 5×11 mm	1
22 µF	10 V	Aluminium electrolytic, radial 5×11 mm	2
47 µF	10 V	Aluminium electrolytic, radial 5×11 mm	4
100 µF	10 V	Aluminium electrolytic, radial 6.3×11 mm	2
100 µF	16 V	Aluminium electrolytic, radial 6.3×11 mm	1
470 µF	10 V	Aluminium electrolytic, radial 8×11 mm	1

For the ceramic capacitors (C104–C128 0.1 µF/25 V): the originals are multilayer ceramic capacitors (MLCC). They do not typically fail with age and replacing them en-masse risks lifting pads. Inspect for cracks (visible as a hairline on the cap body) before replacing. If recapping all 25, use X7R or X5R MLCC with the same 0.1 µF / 25 V or 50 V rating and matching lead spacing.

Polystyrene caps (C302, C305, C312) are stable for the life of the system and typically do not need replacement.

Failure Mode and Symptoms

C322 (470 µF/10 V) — supplies the cassette motor relay. Most likely to dry out and develop high ESR. Symptom: cassette motor will not start, or starts then stops, and the system may print "Read Error a" repeatedly.
C309, C311, C314 (1 µF/50 V) — AY-3-8912 DC-blocking on the three sound output channels. Aged caps cause distorted audio or one of the three channels going silent. Symptom: one or more sound channels distorted or dead.
C315 (100 µF/16 V) — audio output coupling. Aged cap causes weak speaker output. Symptom: quiet speaker, audio jack J103 also affected.
C301, C304 (100 µF/10 V, cassette sub-PCB) — LA4140 audio amp supply rail. Aged caps cause hum, ripple-on-output, or instability. Symptom: mains hum or oscillation on speaker output.
C302 (0.068 µF polystyrene) — reset RC. Failure is rare but produces erratic power-on behaviour. Symptom: random failures to start, requires multiple power cycles.

Diagnostic Procedure

Disconnect the CPC from the monitor (no power).
Open the system unit, separating the keyboard from the lower case.
Disconnect the cassette sub-PCB ribbon at CP001 on the main PCB.
Inspect each electrolytic cap for visible signs of aging: bulged top, leaked electrolyte (brown crust on the PCB around the cap), discoloured plastic sleeve, or cracked seal at the base.
With a multimeter on resistance / ESR mode, probe each electrolytic in-circuit:
Good electrolytic: reads high resistance after a brief charge pulse. ESR within manufacturer spec for the capacitance (e.g. < 1 Ω for 470 µF).

Bad electrolytic: reads low resistance (short), or very high ESR (> 3× the spec value).
If any cap reads bad, remove it and confirm out-of-circuit. Other components on the same rail can give false low-resistance readings.

If the system unit appears generally healthy but the audio is degraded, replace the AY-3-8912 output stage caps (C309, C311, C314, C315) as a set. They are physically clustered on the main PCB and a 4-cap recap takes about 15 minutes.

If the cassette deck does not start the motor, replace C322 first; this single cap accounts for the majority of "cassette motor dead but everything else works" symptoms.

Removal and Replacement

Mark the polarity of the cap on the PCB with a paint pen or photograph.
Apply fresh solder + flux to both leads from the underside of the PCB to wet them and ease desoldering.
Heat one pad and lever the cap up on that side with tweezers. Heat the other pad and lift the cap clear.
Clean both holes with solder wick.
Insert the new cap, matching polarity: the + marking on the cap must match the + marking on the silkscreen (or, equivalently, the negative stripe on the cap must match the position marked 'GND' or 'unmarked' on the silkscreen).
Solder both leads. Inspect for a clean fillet.
Trim leads flush.

The CPC 464 PCB is single-sided with a green solder mask; the pads are not very tolerant of repeated heating. Use a temperature-controlled iron at no more than 350 °C, and limit each desolder cycle to 5–7 seconds. Lifted pads are a known restoration risk and are usually repaired with a fine wire-wrap jumper to the next vias on the trace.

Polarity Reference

The CPC 464 main PCB silkscreen marks the + (positive, rail) side of each electrolytic with a small "+" near one of the two pads. The cap body shows the negative side with a stripe and a "−" symbol. Match these conventions.

Polarity reference convention for through-hole capacitors. The same convention applies to the CPC 464 main PCB and cassette sub-PCB. (Image: minuszerodegrees.net)

Monitor Capacitor Replacement (Brief Pointers)

The CTM-640 / CTM-644 colour monitors and the GT-64 / GT-65 green monitors are the second most-common capacitor-failure site in a CPC 464 setup. Full recap procedures are in the monitor-specific Capacitor Replacement Guides; key points:

Bulk capacitor (typically 100 µF/400 V on the GT-64 / 220 µF/400 V on the CTM-640) holds a lethal charge after the monitor is powered off. Discharge before any work.
Secondary side electrolytics (multiple 47 µF / 100 µF / 220 µF / 470 µF / 1000 µF at 10–35 V) provide ripple smoothing on the +5 V, +12 V and CRT cathode supplies. Recap with 105 °C low-ESR equivalents.
X2-class line suppression capacitor on the mains input may be a RIFA, which is known to crack and fume after 30+ years. Replace with a modern X2-class cap of equal value.
CRT yoke / flyback drive electrolytics: replace only if specific yoke fault is suspected. These caps drive the CRT scan and replacement requires monitor re-alignment afterward.

Post-Recap Verification

Power up the system on the bench with a known-good 5 V/2 A PSU.
Verify the boot screen appears within 2 s.
Run a sound test in BASIC: type "SOUND 1,100" and verify a tone from the speaker and the audio jack.
Insert a known-good test tape and verify load (use the official Amstrad demo tape or a community-archived test image).
Run the keyboard test: type a known sequence and verify all keys produce the expected character.
Run the joystick test (BASIC: PRINT JOY(0)) and verify each direction and fire button.

If any test fails after a recap that previously worked, re-inspect the polarity of every replaced cap before suspecting an actual fault — reversed polarity is the most common error.

When Not to Recap

A blanket recap is not always necessary. If the CPC 464 boots, runs, plays clean audio and loads tapes reliably:

The caps are within tolerance.
Replacement risks introducing solder bridges, lifted pads, and other restoration faults.
Leave the system alone until a specific fault appears.

Always recap if:

Fluid leak visible from any electrolytic.
Audio is degraded or distorted.
Cassette motor fails to start reliably.
System resets randomly under load (sound activity + screen refresh + keyboard activity).

Related Pages

References

Amstrad CPC464 Service Manual (1985, Amstrad Consumer Electronics). Authoritative source for all main-PCB and cassette-sub-PCB capacitor values, designators and IBM/Amstrad service part numbers. The Electrical Parts List occupies pages 17 and 18 of the manual.
Amstrad CPC 464 hardware documentation, Grimware. Reference for the cassette deck function diagram and the relationship between IC301 (audio amp), IC302 (cassette read amp) and the AY-3-8912.